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<title>Hash::Util - A selection of general-utility hash subroutines</title>
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<h1><a>Hash::Util - A selection of general-utility hash subroutines</a></h1>
<p><a name="__index__"></a></p>
<!-- INDEX BEGIN -->
<ul>
<li><a href="#name">NAME</a></li>
<li><a href="#synopsis">SYNOPSIS</a></li>
<li><a href="#description">DESCRIPTION</a></li>
<ul>
<li><a href="#restricted_hashes">Restricted hashes</a></li>
</ul>
<li><a href="#caveats">CAVEATS</a></li>
<li><a href="#author">AUTHOR</a></li>
<li><a href="#see_also">SEE ALSO</a></li>
</ul>
<!-- INDEX END -->
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<p>
</p>
<h1><a name="name">NAME</a></h1>
<p>Hash::Util - A selection of general-utility hash subroutines</p>
<p>
</p>
<hr />
<h1><a name="synopsis">SYNOPSIS</a></h1>
<pre>
<span class="keyword">use</span> <span class="variable">Hash::Util</span> <span class="string">qw(lock_keys unlock_keys
lock_value unlock_value
lock_hash unlock_hash
hash_seed)</span><span class="operator">;</span>
</pre>
<pre>
<span class="variable">%hash</span> <span class="operator">=</span> <span class="operator">(</span><span class="string">foo</span> <span class="operator">=></span> <span class="number">42</span><span class="operator">,</span> <span class="string">bar</span> <span class="operator">=></span> <span class="number">23</span><span class="operator">);</span>
<span class="variable">lock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
<span class="variable">lock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="variable">@keyset</span><span class="operator">);</span>
<span class="variable">unlock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
</pre>
<pre>
<span class="variable">lock_value</span> <span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="string">'foo'</span><span class="operator">);</span>
<span class="variable">unlock_value</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="string">'foo'</span><span class="operator">);</span>
</pre>
<pre>
<span class="variable">lock_hash</span> <span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
<span class="variable">unlock_hash</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
</pre>
<pre>
<span class="keyword">my</span> <span class="variable">$hashes_are_randomised</span> <span class="operator">=</span> <span class="variable">hash_seed</span><span class="operator">()</span> <span class="operator">!=</span> <span class="number">0</span><span class="operator">;</span>
</pre>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p><code>Hash::Util</code> contains special functions for manipulating hashes that
don't really warrant a keyword.</p>
<p>By default <code>Hash::Util</code> does not export anything.</p>
<p>
</p>
<h2><a name="restricted_hashes">Restricted hashes</a></h2>
<p>5.8.0 introduces the ability to restrict a hash to a certain set of
keys. No keys outside of this set can be added. It also introduces
the ability to lock an individual key so it cannot be deleted and the
value cannot be changed.</p>
<p>This is intended to largely replace the deprecated pseudo-hashes.</p>
<dl>
<dt><strong><a name="item_lock_keys">lock_keys</a></strong>
<dt><strong><a name="item_unlock_keys">unlock_keys</a></strong>
<dd>
<pre>
<span class="variable">lock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
<span class="variable">lock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="variable">@keys</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<p>Restricts the given %hash's set of keys to @keys. If @keys is not
given it restricts it to its current keyset. No more keys can be
added. <a href="../../lib/Pod/perlfunc.html#item_delete"><code>delete()</code></a> and <a href="../../lib/Pod/perlfunc.html#item_exists"><code>exists()</code></a> will still work, but will not alter
the set of allowed keys. <strong>Note</strong>: the current implementation prevents
the hash from being bless()ed while it is in a locked state. Any attempt
to do so will raise an exception. Of course you can still <a href="../../lib/Pod/perlfunc.html#item_bless"><code>bless()</code></a>
the hash before you call <a href="#item_lock_keys"><code>lock_keys()</code></a> so this shouldn't be a problem.</p>
</dd>
<dd>
<pre>
<span class="variable">unlock_keys</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<p>Removes the restriction on the %hash's keyset.</p>
</dd>
<dt><strong><a name="item_lock_value">lock_value</a></strong>
<dt><strong><a name="item_unlock_value">unlock_value</a></strong>
<dd>
<pre>
<span class="variable">lock_value</span> <span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="variable">$key</span><span class="operator">);</span>
<span class="variable">unlock_value</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">,</span> <span class="variable">$key</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<p>Locks and unlocks an individual key of a hash. The value of a locked
key cannot be changed.</p>
</dd>
<dd>
<p>%hash must have already been locked for this to have useful effect.</p>
</dd>
<dt><strong><a name="item_lock_hash"><strong>lock_hash</strong></a></strong>
<dt><strong><a name="item_unlock_hash"><strong>unlock_hash</strong></a></strong>
<dd>
<pre>
<span class="variable">lock_hash</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<p><a href="#item_lock_hash"><code>lock_hash()</code></a> locks an entire hash, making all keys and values readonly.
No value can be changed, no keys can be added or deleted.</p>
</dd>
<dd>
<pre>
<span class="variable">unlock_hash</span><span class="operator">(</span><span class="variable">%hash</span><span class="operator">);</span>
</pre>
</dd>
<dd>
<p><a href="#item_unlock_hash"><code>unlock_hash()</code></a> does the opposite of lock_hash(). All keys and values
are made read/write. All values can be changed and keys can be added
and deleted.</p>
</dd>
<dt><strong><a name="item_hash_seed"><strong>hash_seed</strong></a></strong>
<dd>
<pre>
<span class="keyword">my</span> <span class="variable">$hash_seed</span> <span class="operator">=</span> <span class="variable">hash_seed</span><span class="operator">();</span>
</pre>
</dd>
<dd>
<p><a href="#item_hash_seed"><code>hash_seed()</code></a> returns the seed number used to randomise hash ordering.
Zero means the "traditional" random hash ordering, non-zero means the
new even more random hash ordering introduced in Perl 5.8.1.</p>
</dd>
<dd>
<p><strong>Note that the hash seed is sensitive information</strong>: by knowing it one
can craft a denial-of-service attack against Perl code, even remotely,
see <a href="../../lib/Pod/perlsec.html#algorithmic_complexity_attacks">Algorithmic Complexity Attacks in the perlsec manpage</a> for more information.
<strong>Do not disclose the hash seed</strong> to people who don't need to know it.
See also <a href="../../lib/Pod/perlrun.html#perl_hash_seed_debug">PERL_HASH_SEED_DEBUG in the perlrun manpage</a>.</p>
</dd>
</dl>
<p>
</p>
<hr />
<h1><a name="caveats">CAVEATS</a></h1>
<p>Note that the trapping of the restricted operations is not atomic:
for example</p>
<pre>
<span class="keyword">eval</span> <span class="operator">{</span> <span class="variable">%hash</span> <span class="operator">=</span> <span class="operator">(</span><span class="string">illegal_key</span> <span class="operator">=></span> <span class="number">1</span><span class="operator">)</span> <span class="operator">}</span>
</pre>
<p>leaves the <code>%hash</code> empty rather than with its original contents.</p>
<p>
</p>
<hr />
<h1><a name="author">AUTHOR</a></h1>
<p>Michael G Schwern <<a href="mailto:schwern@pobox.com">schwern@pobox.com</a>> on top of code by Nick
Ing-Simmons and Jeffrey Friedl.</p>
<p>
</p>
<hr />
<h1><a name="see_also">SEE ALSO</a></h1>
<p><a href="../../lib/Scalar/Util.html">the Scalar::Util manpage</a>, <a href="../../lib/List/Util.html">the List::Util manpage</a>, <a href="../../lib/Hash/Util.html">the Hash::Util manpage</a>,
and <a href="../../lib/Pod/perlsec.html#algorithmic_complexity_attacks">Algorithmic Complexity Attacks in the perlsec manpage</a>.</p>
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